Nuclear magnetic resonance imaging (MRI) apparatuses are diagnostic imaging apparatuses for medical use that induce nuclear magnetic resonance in nuclei of hydrogen atoms contained in an arbitrary plane traversing a test subject, and produce a tomographic image of a region in the plane using generated nuclear magnetic resonance signals. In general, when a slice magnetic field gradient for specifying imaging slice is applied, an excitation pulse is applied in order to excite magnetizations in the plane at the same time. Nuclear magnetic resonance signals (echoes) generated in the course of the precession of the magnetizations excited by the pulses. Positional information is imparted to the echoes by the magnetic field gradient, and Fourier transform of the echoes is performed to reconstruct images. The angle of inclination of the magnetization with respect to the direction of the static magnetic field, i.e., flip angle, is determined by time integration value of amplitude of the excitation pulse, and an angle providing appropriate image contrast is chosen according to the imaging method.
The pulse and magnetic field gradients used for generating echoes are applied according to a predefined pulse sequence. As for the pulse sequence, various pulse sequences are known in association with different purposes. For example, a gradient echo (GrE) type high-speed imaging method is a method in which such a pulse sequence is repeatedly executed, and the phase-encoding magnetic field gradient is sequentially changed for every repetition to sequentially measure echoes in a number required for obtaining one tomographic image.
Another GrE type pulse sequence is a phase compensation type pulse sequence. In this pulse sequence, a magnetic field gradient pulse for making the time integration value of the magnetic field gradient for each axis zero is added to GrE. Degree of the flip angle is generally larger than that used in the GrE method, and the phase thereof is reversed in every execution. Moreover, the repetition time TR is shorter, and is around 5 ms.
In such a GrE type imaging method, before execution of a pulse sequence for measuring echoes required for image reconstruction (imaging mode), magnetization is repeatedly excited in order to obtain a steady state of magnetization. This procedure is called non-imaging mode. In the non-imaging mode, the same pulse sequence as that used in the imaging mode is executed a predetermined number of times without measuring echoes. However, in order to obtain a steady state of magnetization with less times of excitation, the flip angle in the non-imaging mode may be gradually increased from a small angle and made closer to the angle used in the imaging mode.
Moreover, in these high-speed imaging methods, the flip angle greatly influences image contrast. Therefore, an angle providing a certain degree of image contrast is generally chosen from the range of 10 to 90 degrees as the flip angle for the imaging mode, and the flip angle is not usually changed during a period for obtaining one image.
In MRI, the magnetic resonance frequency becomes higher in proportion to the magnetic field intensity. In connection with this fact, there arises a problem of increase in absorption of RF electric power into human bodies, called specific absorption rate (SAR), and development of countermeasure against it constitutes a subject of researches. SAR is RF irradiation power per unit time, and it is proportional to the time integration value of square of the flip angle and is in inverse proportion to TR. The reference value of the maximum thereof for human body is defined to be 4 W/kg. When a GrE type pulse sequence is used, RF irradiation is repeated in a short time, and therefore SAR becomes large. In particular, phase compensation type GrE pulse sequences use a short TR and a large flip angle, and therefore it is difficult to apply such sequences to a human body in a high magnetic field apparatus using a magnetic field of about 3 Teslas or more in view of safety. For example, for a case that a phase compensation type GrE pulse sequence using a flip angle of 60 degrees and TR of 3 ms is executed in an apparatus using a magnetic field of 3 Teslas, SAR is calculated to be 4.7 W/kg. This value exceeds the reference value, and therefore it is impossible to perform imaging.
To solve this problem, there has been proposed a method of changing flip angle of RF excitation pulse for the imaging mode according to the amount of phase encoding so that image contrast should not be degraded, in consideration of the specific absorption rate SAR (henceforth this method is referred to as prior art 1, Patent document 1). This method is based on the fact that image contrast in MRI is generally determined by contrast of echoes having a small phase encoding amount, and the flip angle is made large when the phase encoding amount is small, so that image contrast should not be degraded even when the flip angle is changed.
Patent document 1: Japanese Patent Unexamined Publication (KOHYO) No. 2005-524453